Low-Cost Measurement for a Secondary Mode S Radar Transmitter

Low-Cost Measurement for a Secondary Mode S Radar Transmitter Angel Parra-Cerrada, Vicente Gonzalez-Posadas, Jose Luis Jimenez-Martin, Alvaro Blanco-del-Campo, Wilmar Hernandez, Senior Member, IEEE, and Carlos Calderon-Cordova

Abstract—A low-cost, multiple-purpose, and high-precision MSSR [3]. However, these papers do not deal with the up-link timing test setup for the measurements of secondary signal of the MSSR taking into consideration the modulation Mode S radar transmission signal was proposed. The goal performances of this signal. This paper 34 shows a detailed was to fully guarantee compliance of the proposed transmitter under test with the really hard International Civil Aircraft analysis of the transmitted signal by an MSSR, allowing Organization requirements using traditional measurement us to verify compliance with International Standards and equipment, which was difficult or even impossible to ensure Recommended Practices [4]. This verification is made by using up to now. The low-cost structure proposed in this paper a exible user-controlled method that is independent of any allows the user to perform measurements independently of manufacturer of specific commercial test equipment, which the measurements performed by the pieces of test equipment shelled by the manufacturer of radar, which is a very important in many cases is the same as the manufacturer of the radar. aspect since the independence of the verifications is a mandatory This method guarantees that problems cannot be undetected requirement established by the safety standards of civil aviation. because of the same implementation of some hardware is used The proposed setup has been used to verify several transmitters in transmitter and test equipment. with some defects that are not detected by monopulse secondary surveillance radar specific pieces of test equipment that are Nowadays, the number of MSSR is increasing very sig focused on more high-level functionalities. It also is valid and it nificantly and a further increase is expected in the coming has been used, as a general-purpose setup, for testing other radio years [5]. In addition, due to the fact that the MSSR is a navigation aids. critical point in aviation safety, it is of utmost importance to Index Terms—Commercial-off-the-shelf (COTS) equipment, have diagnostic capabilities of the MSSR independently of differential phase shift keying, envelope and phase measure the manufacturer of the radar [6] and as a complement to the ments, global positioning system disciplined oscillator (GPSDO), built-in test equipment of the radar, having a low-cost system Mode S radar, monopulse secondary surveillance radar (MSSR), that uses light and easily transportable equipment that allows radar measurements, transmitter measurements. checking in situ in remote areas in an affordable manner.

I. INTRODUCTION During the fifties, the United States of America required all aircraft entering its airspace to be identified by means of sec N THIS paper a low-cost and high-precision method ondary surveillance radar (SSR), compatible with the military to measure and analyze the most important parameters I identification friend or foe (IFF) system IFF Mark X selective of the transmitter of a monopulse secondary surveillance identification feature system [7]. Due to its widespread use, radar (MSSR) is presented. In the scientific literature, there it was established as a reliable radar sensor for air traffic are several papers that analyze the performance of monopulse control (ATC), although with some technical limitations, such receivers [1], MSSR antennas [2], and spurious emissions of an as garble, fruit, track wander, and multipath propagation. For this reason, some improvements were made, for example, monopulse in azimuth (eliminating track wander) [8], greater This work was supported in part by the Spanish Ministry of vertical aperture antennas (reducing multipath cancelation and science ana Education under Project TEC2006-13248-C04-04/TCM, Project Consolider CSD2008-0068, and Project TEC2009-14525-C02-01, in part by silence cone) [9], and sidelobe suppression system improve the National Secretary of Higher Education, Science, Technology and Inno ment. All these improvements allowed this ATC system to vation of Ecuador through PROMETEO Project, and in part by the Technical become the most world-wide used one [10]. Nevertheless, once University of Loja, Loja, Ecuador. The Associate Editor coordinating the review process was Dr. Matteo Pastorino. these improvements were achieved, more improvements in A. Parra-Cerrada, V. Gonzalez-Posadas, and J. L. Jimenez-Martin are SSR systems were required, such as selective modes of inter with the Department of Teoria de la Serial y Comunicaciones, Universidad rogation and getting a better data link between radar ground Politecnica de Madrid, Madrid 28040, Spain (e-mail: [email protected]; station and aircraft [11]. These improvements were achieved [email protected]; [email protected]). A. Blanco-del-Campo is with the Microwave Sensing, Signals and by modulating not only amplitude of the transmitted uplink Systems Group, Department of Microelectronics, Delft University of signal but also its phase. This way, Mode S radar was on track. Technology, Delft 2628CC, The Netherlands (e-mail: a.blancodelcamp® tudelft.nl). Furthermore, as it was impossible to renew at once all the W. Hernandez and C. Calderon-Cordova are with the Department systems that were already in use worldwide, the new system of Computer Science and Electronics, Universidad Tecnica Particular had to be capable of responding and transmitting to older de Loja, Loja 11432, Ecuador (e-mail: [email protected]; [email protected]. ec). versions of IFF, and this is the reason why Mode S radars transmitted signals and systems themselves became more and more complicated, fulfilling hard requirements in amplitude, RF GENERATOR phase, and time synchronization as well. It is a well-known 10 MHz fact that whatever system is used to control, manufacture, (^) 970 MHz REFECENCE or develop anything in which human lives can be involved and it has to get through a very strict certification process. Therefore, amplitude, phase, frequency, and timing conditions MODES OSCILLOSCOPE TX ^ of Mode S radars must be measured and certified [12], involving specific and quite expensive equipment as well as GPIB/ETH high technical staff to measure them. The manufacturer is usually the responsible of the certification process, but each MATLAB piece of equipment must be verified periodically by the user as it is required by each national ATC normative.This is normally Fig. 1. Schematic of the proposed test setup. requires once a year. This paper emphasizes this key point and shows a novel method of Mode S radar ground transmitter mea surements using only the low-cost commercial-off-the-shelf stable reference signal has to be used for accurate phase and equipment. The proposed method provides a complete timing measurements. verification of the transmitter using a typical acquisition To meet the first condition, this paper presents the scheme. So, this verification is performed without any test setup shown in Fig. 1, which uses the digital dependence with the radar manufacturer. In short, for tim oscilloscope TDS3034C as a data acquisition system. ing requirements, a global positioning system (GPS) dis Its sampling rate (2.5 G/s) is high enough to manage the ciplined oscillator (GPSDO) is used [13], and for sam required bandwidth and to store the needed information for a pling and storing the signal for proper automatic processing, simple and unique interrogation. a high-frequency sampling oscilloscope is used [14]. To meet the second condition, a GPSDO is used. This In addition, a computer is used to get the data from the is shown in Fig. 2 and its performance is explained oscilloscope for analysis and signal processing. The processing in Section II-B. of the signal on a computer using MATLAB allows the The test setup shown in Fig. 1 uses an intermediate specific and particular parameter monitoring that each user frequency (IF) of 60 MHz. This frequency is a typical IF for desires independent of the interpretation that the manufacturer SSR systems and is low enough to be used in many medium- of the transmitter made (usually, it is the same interpre range digital oscilloscopes. Being a typical frequency makes tation that the manufacturer implements on the COST test it easy to purchase filters, attenuators, mixers, and so on at equipment). low prices. A prototype of the measurement system was implemented Using a digital oscilloscope along with a computer to and it was used to carry out the measurements and the analysis control it and process the signal becomes a low-cost and of some signals of two defective transmitters. high-performance signal analysis system, good enough for The organization of this paper is as follows. The test setup is our purposes. Therefore, the proposed test setup for the explained in Section II. The signal acquisition and processing Mode S radar transmission measurements consists of the is explained in Section III. Section IV is aimed at showing following: 1) attenuator; 2) passive mixer; 3) low-pass filter; the experimental results. Finally, the main conclusions of this 4) 10-MHz GPS oscillator reference; 5) signal generator; paper are summarized in Section V. 6) digital oscilloscope; 7) desktop computer; and 8) MATLAB. At this point, it is important to say that direct conversion of II. TEST SETUP the IF signal is being used because of its inherent advantages Usually, when trying to measure a waveform that is difficult compared with the use of a traditional I/Q (or I/Q and video) to measure, the required and, therefore, used equipment is demodulator used in most transponders. Some of the main specifically designed for that purpose, and very often, the advantages are that amplitude to I/Q time delay, amplitude state of the art technology is involved in it. For this reason, mismatch between I/Q branches, and I/Q phase unbalance equipment designed for this purpose is hard to find and errors are avoided. expensive. However, sometimes designers can use inexpensive technologies to solve the problem of measuring waveforms with very complex characteristics [15], [16]. For example, not A. Signal Parameters long ago, if real-time data acquisition and signal processing As it has been said before, when Mode S radars are was involved, it was difficult and very expensive to save a used, human lives are involved and security becomes a large amount of data. But a wide range of digital oscilloscopes must. Thus, the requirements of the signal parameters of a nowadays can do it [17]. Mode S interrogation are difficult to meet, to ensure proper To carry out Mode S radar transmitter measurements, operation and that the signal complies with the International two main conditions have to be met. First, a bandwidth Civil Aircraft Organization (ICAO) [4] requirements. of 70 MHz (IF + BW/2) has to be sampled and stored, These parameters do not only define spectrum and amplitude, although offline processing can be used. Second, a highly but also intrapulse phase evolution, amplitude variation in RF SIGNAL E EEC1 6 E EEL' 0\ GENERATOR t LEES „_ ._

TONE MODES (970 MHz) TRANSMITTER

ATTENUATOR

RF SIGNAL MIXER LOW PASS FILTER (1030 MHz) • CONTROL • DATA TRANSFER

• PRESENTATION I • DATA ANALISYS

Fig. 2. Test setup. each data bit, pulse duration, and so on. The parameters that The short-term stability is given by the crystals of quartz or define a Mode S interrogation are shown in Table I. rubidium [18], [19], and it archives a frequency stability much Most tight requirements are related to timing conditions. better than the one required, 0.01 MHz at 1030 MHz, for a Timing parameters are defined for amplitude (e.g., rise time 10-MHz reference signal. and width) and amplitude to signal phase, and also for phase The precision requirements defined by ICAO are in terms evolutions. Measurements with a precision of nanoseconds are of frequency stability and position of data bits. The tolerance required because the tolerances are of the order of tens of in frequency defined by ICAO is ±0.01 MHz @ 1030 MHz, nanoseconds, so timing setup becomes a very hard task to and this is 9709 ppm. The requirement of position of the bits accomplish. is ±20 ns at a maximum distance of 28 /us with 714286 ppm. It is important to remark that all pieces of commercial off- Therefore, the most restrictive factor is frequency stability. the-shelf equipment known by the authors are mainly focused To measure the frequency with an uncertainty of 1% at the in the upper level information and operation of the MSSR. limit of the acceptable range defined by ICAO, it is required They are able to generate scenarios, targets with movement, a stability of the system better than 97 ppm. The tolerance in garbling, and so on. But these pieces of test equipment use a timing is ±20 ns, so an oscilloscope with a sample rate of demodulator to decode the interrogations and most of them 2.5 GSampes/s provides a 2% accuracy. A higher sampling are not able (really none of the equipment known by the rate could be used in the case of requiring a more selective authors) to detect problems in the modulation waveform. system for measuring transmitters near the specification limit. Therefore, they are able to decode the signal in a high S/N The test setup does not require a phase-locked loop (PLL) condition, but they do not guarantee a proper decodification to lock the frequency because signal requirements are referred by the transponders of the aerial platforms in a realistic S/N to nominal transmitter frequency. Thus, a GPSDO is used to condition. obtain an absolute 10-MHz reference. Most of the systems for aircrafts do not use a PLL to lock the signal, and instead, they use noncoherent receivers, because they must listen to multiple B. Timing Setup signal sources at the same time, replying to the highest priority In this paper, a GPSDO was used to obtain a highly stable one. Most systems use the arrival time as priority policy, and reference signal. The principle of operation of a GPSDO is they reply to the first one and the others are thrown away. as follows. A GPSDO works by disciplining or steering a But in some more complex systems, those that include military high-quality quartz or rubidium oscillator by locking the functionalities, other policies are used to determine the reply output to a GPS signal via a tracking loop mainly in two ways: priority and order. either through a loop filter or via a microcontroller using As most of the signal parameters defined by ICAO [4] are software to compensate not only for phase and frequency related to the phase of the signal, the accuracy of measurement of the signal but also for aging. Thus, stability in the long is improved using a GPSDO as reference signal (10 MHz) for term and in the short term of the GPS signal is combined. the generator (970 MHz) and also for the oscilloscope that Spectrum and Carrier Acquired Frequency Signal FFT{} &£& T

H-1„ti„(j<») Phase{} log{}

Phase-Reversal Detector

Phase- Reversal Pulses Location and Width and Length Location

Pulses Amplitude, Data Validation CRC Rise-Time, and Fall-Time Data Location Phase Variation Amplitude Variation P6 Phase Variation

Fig. 3. Signal processing scheme.

TABLE I MODE S INTERROGATION WAVEFORM PARAMETERS

PARAMETER NOMINAL VALUE TOLERANCE PI and P2 pulse Width 800 ns ±100 ns 16.25 /is (Short) P6 Pulse Width ±250 ns 30.25 (is (Long) PI to P2 Space 2 (is ±50 ns Pulse Rise Time - 50 ns to 100 ns Pulse Fall Time - 50 ns to 200 ns P2 Rise to Phase Reverse 2.75 fj,s ±20 ns P6 Rise to Phase Reverse 1.25 fj,s ±20 ns Phase Change Duration - < 80 ns Data phase reversal (Nx250ns) (N<2) ±20 ns Guard time after last data - > 500 ns P6 Amplitude > (PI Amplitude) - 0.25 dB (1st ^s ) - P6 Amplitude Variation ±1 dB (Total) - P6 Amplitude Variation ±0.25 dB (between successive chips) - P6 Maximum phase variation - < 370° Phase variation per chip 180° ±5° Frequency 1030 MHz ±0.01 MHz samples and stores the transmitted signal. The test setup is baseband, where the real and imaginary parts of the resulting shown in Fig. 2. complex signal are extracted

III. SIGNAL PROCESSING / = ?lt(S(t)e-jcoot) (1) In this paper, the oscilloscope only acquires the 60-MHz Q = %(S(t)e-Jmot) (2) IF signal and all the signal processing stage is carried out on a personal computer using MATLAB. First, the sampled and where stored signal is corrected due to the fact that an attenuator has m = lit • 60 • 106 rad/s. been placed before the mixer, which was done to protect it 0 from high-power signals generated by the Mode S interrogator The obtained I/Q signals can be low-pass filtered module. After that, the spectrum is obtained through an FFT, and subsampled without degradation of the information and the carrier frequency value and the spectrum mask [4] or signal shape. To subsample the signal reduces the are checked. Next, the signal is digitally downconverted to required computational burden. The RF bandwidth defined by 0.8 £ 0.6 to 0.4 nc i H •D n? 3 o 0 to tco E -uv o TJ -0 4 E H -0.6

-0.8 4 6 8 Frequency (Hz) x10 Time (s x10 Fig. 5. Spectrum of the laboratory generator short Mode S interrogation Fig. 4. TimTin e domain of the laboratory generator short Mode S interrogation signal. signal.

ICAO is 8 MHz, so a 10-MHz low-pass filter is used to filter The IF signal is directly acquired using the oscilloscope and baseband I/Q signals. This filter does not affect the spectrum its time-domain representation is shown in Fig. 4. of the signal of the transmitter and it is able to eliminate the This signal is processed, the signal spectrum is obtained, undesired mixers. and it is compared with the spectrum mask defined by Mode S regulation defines the amplitude requirements in ICAO (Fig. 5). decibels and the phase ones in degrees. Thus, a logarithmic All the parameters of the signal, including the phase evo amplitude detector is used for the amplitude (video signal), lutions, are obtained using MATLAB and the information while inverse tangent of I/Q for the phase (phase signal). data are decoded. With the signal envelope, the pulse posi The video signal is used for pulse measurements (i.e., width, tion and the rise and fall times are checked. The phase relative position, length, amplitude, rise time, and fall time) reversal is detected from the phase evolution and the data and the phase signal is used for the phase-reversal position extraction is synchronized with its position. The envelope and length, allowing data extraction and validation. of the signal and the data detection samples are shown The signal processing scheme is shown in Fig. 3. By achiev in Fig. 6. ing a complete detection and demodulation, all the parameters A more detailed image of the phase of P6, including the defined by ICAO (see Table I) can be measured and checked sampling points, is shown in Fig. 7. The first vertical line is with the required precision and tolerance, and this can be the phase-reversal detection, and all the following ones are the carried out by using general-purpose equipment. data sampling points. The greatest advantages of this scheme (see Fig. 3) are that The polar representation of the P6 I/Q evolution is shown the measurement process is carried out fully automatically and in Fig. 8. This representation provides a good visual overview that it can perform any specific measurement defined by the of the signal quality. user. The system can be configured for measuring, checking The signal from the high-quality laboratory generator parameters, and storing the results as long as the user wants. complies with all the parameters defined for Mode S inter Furthermore, if a temperature or pressure chamber is available, rogation and the obtained values are in accordance with the tasks such as controlling the chamber by a personal computer programmed ones. As a summary of the signal analysis, some and performing automatic measurements, while the ambient parameters are pointed to be compared with the ones of the conditions are controlled, can also be conducted. faulty transmitter: IV. EXPERIMENTAL RESULTS 1) P6 amplitude variation: 0.41 dB; The test setup presented in this paper has been used to 2) phase reverse duration: 75 ns; test three different models of Mode S interrogator trans 3) maximum phase variation in P6: 365°; mitters. The first one is a high-quality laboratory generator, 4) data-phase position error: 1 ns; and the second ones are actual Mode S transmitters that 5) maximum amplitude variation per chip: 0.09 dB; have some well-known problems on their signal synthesizers. 6) center frequency: 60.000 MHz. These models are used to provide valid and nonvalid signals to check and validate the test setup. B. Actual Mode S Transmitters With Some Problems A. High-Quality Laboratory Generator on Their Signal Synthesizers The acquired data of the first Mode S transmitter, which The signal from the first defective Mode S transmitter was complie with all ICAO [4] requirements, are described in the acquired using the proposed test setup and processed using following lines. MATLAB. Some of the most relevant information is shown CO 0

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Fig. 6. Laboratory generator signal envelope, I/Q, and phase.

400

350

300

250

« 200 Q. CO o- 150 180 100

0 3 4 Sampling points x10 270 Fig. 7. Laboratory generator P6 phase.

Fig. 8. Laboratory generator P6 I/Q. in Figs. 9 and 10. The envelope amplitude of the signal and the phase are shown in Fig. 9. In the detailed phase marked as fault: evolution of the data area of the signal (P6) (Fig. 10), it can 1) P6 amplitude variation: 1.81 dB (FAULT); be observed that this modulator presents a problem in the 2) phase reverse duration: 118 ns (FAULT); phase response. 3) maximum phase variation in P6: 404° (FAULT); The phase signal shown in Fig. 10 has phase transition 4) data-phase position error: 15 ns (OK); problems such as overshooting and frequency error as well. 5) maximum amplitude variation per chip: 0.7 dB The polar representation of the P6 I/Q evolution is shown in (FAULT); Fig. 11. It shows the phase error in bits and the envelope 6) center frequency: 60.006 MHz (OK). variation of the signal. Other defective Mode S transmitter resulted in the phase After the full analysis of the transmitted signal of shown in Fig. 12. Some problems with phase transitions may the Mode S transmitter under test, the parameters of be detected with the signal spectrum. But others like the one the ICAO specifications were measured accurately. shown in Fig. 12 can only be detected by conducting a more The parameters that did not meet the specifications are complex phase analysis. The signal met all the requirements m 0 I i I 1 1 V iWfw^'W o -100 rri

-200 I I I I I I I 0 0.5 1 1.5 2 2.5 3 3.5 4 x10"5 0.05 1 1 ^-^JMIIJJIJ^ iriMIMiliiNL ° -0.05 I I I I I I I 0 0.5 1 1.5 2 2.5 3 3.5 4 x10"5 0.05 I i 1. 1 1

-0.05 I i 1 1 1 0 0.5 1 1.5 2 2.5 3 3.5 4 x10"S 100 I i 1 1 1 50 0 - -50 i i 1 1 1 0.5 1.5 2 2.5 3.5 Time (s) x10

Fig. First Mode S transmitter signal envelope, I/Q, and phase.

P6I/Q 90 40

180

1000 2000 3000 4000 5000 6000 7000 8000 9000 Sampling points

Fig. 10. First Mode S transmitter P6 phase. 270 but maximum phase variation in P6. In spite of the fact that Fig. 11. First Mode S transmitter P6 I/Q. this signal was generated at nominal frequency, it presented spectrum degradation due to the fact that the phase derivation in the phase changes was always negative. If the phase of the on the market. Some examples of these high-end equipment signal is not studied in detail, this problem can be interpreted are the following: Radar Analysis Support System, with a cost as a problem with the collector modulation, because most of of more than € 1 000 000; and high-performance signal and the demodulators are able to demodulate this defective phase spectrum analyzer, with a cost of more than € 75 000. modulation providing correct data. When a demodulator is Table II shows that the cost of the system presented working with a high S/N ratio, as the high-level test equipment in this paper is less than € 31000. This cost compares does, the signal is correctly demodulated, but it cannot assure very favorably to the above-mentioned pieces of high-end that the signal will be correctly demodulated by a receiver in equipment that are available on the market. To evaluate the real media S/N conditions. presented cost, it must be taken in consideration that the list of pieces of equipment presented in Table II are general-use C. Cost of the Proposed System equipment. Therefore, the budget is shared with other test Some pieces of high-end equipment, which are designed purpose, but the pieces of high-end equipment are only for with a similar purpose to the one presented in this paper, exist the specific test purpose they are designed. 3500 To illustrate the use of the proposed measurement setup, the results of the performance of two Mode S interrogator 3000 transmitters have been shown. The measurement of a reference signal from a laboratory generator with known parameters 2S00 confirms the accuracy of the proposed setup.

IT 2000 A set of measurements of two Mode S transmitters with nu some problems was shown and a correct demodulation of the data could be obtained with some demodulator schemas, but the proposed technique allows one to both detect 1000 and successfully measure the problems presented by the transmitter under test. Finally, the authors of this paper would 500 also like to say that the proposed method presented in this

Q| 1 HI I I-L- paper has been successfully used to analyze and verify the 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 signals transmitted by the aviation transponder interrogation Sampling points Mode 5, distance measuring equipment, and Doppler very Fig. 12. Second Mode S transmitter phase evolution of P6. high-frequency omnidirectional range equipment.

TABLE II ACKNOWLEDGMENT COMPONENT PARTS OF THE PROPOSED SYSTEM The authors would like to thank M. A. del Casar Tenorio | Description || Price | for his personal and great contributions to the development of Coaxial Low Pass Filter BLP-100+ 28 Euros this paper. GPS. Spectracom Epsilon Clock Model EC1S 2,130 Euros Coaxial Frequency Mixer ZAD-11+ 53 Euros REFERENCES Digital Oscilloscope TEKTRONIX TDS3034C 9,610 Euros [1] C. Jacovitti, "Performance analysis of monopulse receivers for secondary Signal generator R&S SMBV100A 16,443 Euros surveillance radar," IEEE Trans. Aerosp. Electron. Syst, vol. AES-19, RF Attenuator NARDA SA12N350A-40 1,350 Euros no. 6, pp. 884-897, Nov. 1983. Matlab, Standard License 2,000 Euros [2] J. A. Lord, G. G. Cook, A. P. Anderson, and R. S. Orton, "Diagnostics of Personal computer 400 Euros an SSR array antenna from the measured near-field intensity using phase Total Cost: 30,664 Euros retrieval," in Proc. 8th Int. Conf. Antennas Propag., vol. 1. Edinburgh, U.K., Mar./Apr. 1993, pp. 315-318. [3] T. Ikaheimonen, "Measurement of radar spurious emission with high dynamic range and optimized measurement time," IEEE Trans. Instrum. V. CONCLUSION Meas., vol. 60, no. 3, pp. 1010-1016, Mar. 2011. [4] International Standards and Recommended Practices—Annex 10 The proposed test setup provides a low-cost and to the Covention on International Civil Aviation—Aeronautical high-performance system to test and validate Mode S transmit Telecommunications: Surveillance Radar and Collision Avoidance Systems, 4th ed., ICAO, Jul. 2007. [Online]. Available: ter performances. The proposed technique is able to cover all http://storel.icao.int/index.php/publications/annexes/10-aeronautical- the ICAO requirements of the modulated signal into a single telecommunications/annex-10-volume-iv-surveillance-radar-and- tool. The setup is able to analyze all Mode S interrogation collision-avoidance-systems-english-printed-11331 .html [5] J. Tol, P. van Genderen, and L. P. Ligthart, "Improvement of the SSR parameters on an automatic way and perform statistic study mode S data link using a distributed ground system," in Proc. CIE Int. of parameters. Conf. Radar, Beijing, China, Oct. 1996, pp. 519-522. Most or even all the pieces of high-level test equipment [6] I. Balajti, "Performance measurements of the radar 'in situ,"' in Proc. Microw., Radar Remote Sens. Symp. (MRRS), Kiev, Ukraine, Sep. 2008, for MSSR test purpose are focused on operational perfor pp. 334-339. mances of the radar and they have a demodulator to decode [7] R. M. Trim, "Mode S: An introduction and overview," Electron. the transmitted signal. They work with an extremely high Commun. Eng. J., vol. 2, no. 2, pp. 53-59, Apr. 1990. S/N ratio, so they cannot assure proper demodulation in [8] D. Bonefacic, J. Jancula, and N. Majurec, "Model of a monopulse radar tracking system for student laboratory," Radioengineering, vol. 16, no. 3, a real operational S/N and interference environment. The pp. 62-67, Sep. 2007. proposed test setup is oriented to analyze in detail the signal [9] L. Sorokosz, W. Zieniutycz, M. Pergol, and M. Mazur, "Mutual coupling to assure that it has all the robustness, as it is required between IFF/SSR microstrip antennas with reduced transversal size— Experimental study," Radioengineering, vol. 20, no. 1, pp. 284-288, by ICAO. Apr. 2011. The use of a direct IF signal conversion avoids timing [10] V. A. Orlando, "The mode S beacon radar system," Lincoln Lab. J., calibration and increases the test simplicity, maintenance, and vol. 2, no. 3, pp. 345-362, 1989. [11] PR. Drouilhet, Jr., "The development of the ATC radar beacon system: precision. All the processing power of an analysis tool such Past, present, and future," IEEE Trans. Commun., vol. 21, no. 5, as MATLAB is available for any detailed analysis or signal pp. 408^121, May 1973. modeling that is required. The high acquisition frequency [12] Federal Aviation Administration - Advisory Circular N. 120-86. Aircraft Surveillance Systems and Applications. [Online]. Available: used, 2.5 G/s, provides a 0.4-ns precision on all timing http://www.faa.gov/documentLibrary/media/Advisory _Circular/AC% parameter and guarantees this precision for all amplitude to 20120-86.pdf. phase measurements without any degradation due to the use [13] C. L. Cheng, F. R. Chang, L. S. Wang, and K. Y. Tu, "Highly-accurate real-time syntonization by use of a single-frequency receiver with GPS of parallel equipments such as a power meter and a differential carrier phase," in Proc. 18th Eur. Freq. Time Forum (EFTF), Guildford, phase shift keying demodulator. U.K., Apr. 2004, pp. 159-166. [14] R. Kraus, M. Treanor, W. J. Sarjeant, and R. Dollinger, "Programmable instrumentation for high repetition rate experiments providing control in real time and acquisition of a train of pulses," Rev. Sci. Instrum., vol. 59, no. 7, pp. 1226-1231, Jul. 1988. [15] H. S. Mir and L. Albasha, "A low-cost high-performance digital radar test bed," IEEE Trans. Instrum. Meas., vol. 62, no. 1, pp. 221-229, Jan. 2013. [16] L. A. Muth, C. M. Wang, and T. Conn, "Robust separation of background and target signals in radar cross section measurements," IEEE Trans. Instrum. Meas., vol. 54, no. 6, pp. 2462-2468, Dec. 2005. [17] M. Yeary et al, "Multichannel receiver design, instrumentation, and first results at the national weather radar testbed," IEEE Trans. Instrum. Meas., vol. 61, no. 7, pp. 2022-2033, Jul. 2012. [18] R. E. Updegraff, "Precise time and frequency control using the Navstar GPS and a disciplined rubidium oscillator," in Proc. 2nd Int. Conf. Freq. Control Synth, Leicester, U.K., Apr. 1989, pp. 92-96. [19] R. M. Hambly and T. A. Clark, "Critical evaluation of the Motorola M12+ GPS timing receiver vs. the master clock at the United States Naval Observatory, Washington, DC," in Proc. 34th Annu. PTTIMtg., Reston, VA, USA, Dec. 2002, pp. 109-115.